1. Field of the Invention
The present invention relates to a convergence correction apparatus for use in a color television receiver, and more particularly to a convergence correction apparatus capable of performing an automatic convergence correction with high accuracy by digitally controlling various correction waveforms.
2. Description of the Prior Art
Generally in a video projector, three projection tubes are used for applying light beams of three primary colors red (R), green (G) and blue (B) for expanding and projecting a video image picture onto a screen, where the incident projection angles (hereinafter, referred to as, "convergence angle") of the light beams from the projection tubes with respect to the screen are different from each other, resulting in that color deviation, i.e., misconvergence may occur in the picture displayed on the screen. Convergence correction to such misconvergence has been implemented by such a system that an analog-like correction waveform is created in synchronization with horizontal and vertical scanning periods so that the amplitude and shape of the analog-like correction waveform are varied to be adjusted for the convergence correction. However, such a conventional system has been accompanied by a problem in the viewpoint of correction accuracy.
The conventional adjustment system mentioned above has been also accompanied by another problem that much time is required in operation because the correction is manually performed while visually observing the misconvergence on the screen.
In order to solve this problem, the Japanese Patent Unexamined Laid-Open No. 8114/1984 discloses a digital convergence apparatus capable of convergence correction with high accuracy. Besides, the Japanese Patent Publication No. 38797/1991 discloses a digital convergence apparatus for performing an automatic correction. The Japanese Patent Publication No. 5314/1992 discloses a digital convergence apparatus capable of coping with various scanning frequencies. The Japanese Patent Unexamined Laid-Open No. 11388/1987 discloses a digital convergence apparatus capable of coping with various aspect ratios. The Japanese Patent Unexamined Laid-Open Nos. 122417/1977, 147016/1977, and 79893/1985 each discloses a method of performing a convergence correction by dividing an analog waveform.
One of such conventional convergence correction apparatuses for use in a video projector is described below reference to FIG. 26.
Referring to FIG. 26, the apparatus includes a projection cathode-ray tube 8, a convergence yoke 9, a deflection yoke 10, a screen 86, and a projection lens 87.
A video signal is applied to a video circuit 81 through a video input terminal 1, where the input video signal is amplified to an appropriate amplitude to drive the projection cathode-ray tube 8. The video circuit 81 serves as the conventional television receiver in the normal mode, whereas it displays, as shown in FIG. 27, on the display screen a crosshatch signal fed from a digital convergence circuit 82 during a convergence adjustment operation mode.
In the digital convergence of the crosshatch signal, correction data of adjustment points displayed on the screen is stored and interpolation of the correction data is performed among the adjustment points to create a correction waveform. Accordingly, correction can be effected for each adjustment point independently, so that high-accuracy correction can be implemented. Electron beams are scanned by means of a deflection circuit 83 and the deflection yoke 10 in the projection cathode-ray tube 8 in accordance with a sync. signal applied through a sync. signal input terminal 2. Although a single projection cathode-ray tube 8 is shown in FIG. 26, normally used are three projection cathode-ray tubes for primary three colors of R, G and B.
An adjustment pattern detector 85 such as a camera performing photodetection, detects a crosshatch pattern projected on the screen 86. The detection signal of the crosshatch pattern is fed from the detector 85 to an adjustment-point detector circuit 84. The adjustment-point detector circuit 84 detects convergence deviation of each adjustment point and adjusts the amount of correction of the digital convergence circuit 82 according to the resultant detection signal, thus automatically accomplishing the convergence adjustment.
As described above, by detecting the misconvergence in the picture displayed on the screen with the detector camera 85 and using the resulting detection signal to control correction data, there can be achieved an automatically adjustable high-accuracy convergence correction apparatus.
In such a conventional convergence correction apparatus as described above, it is required to input misconvergence correction data of each adjustment point in each mode in order to cope with signal sources having different input scanning frequencies and different aspect ratios. Therefore, the conventional apparatus has such problems that it requires much time for the adjustment and significantly increased memory capacity and circuit scale due to the necessity of a particular adjustment function for each mode.